<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        div {
            width: 300px;
        }

        .aside {
            width: 100px;
            height: 150px;
            float: left;
            background: black;
        }

        .main {
            /*形成BFC*/
            /*overflow:hidden;*/

            height:200px;
            background-color:red;
        }
    </style>
</head>
<body>
<div class="aside"></div>
<div class="main"></div>
</body>
</html>